Xerographic development method

ABSTRACT

A cascading development method employs a developer unit which is equipped with a developer pool at the upper portion of the development zone and a development electrode at the lower portion that provides high development efficiency, good solid area coverage and reduced background density.

[4 1 Feb. 11, 1975 United States Patent [191 Jo et al.

'Weiler Berlier et al.

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Primary Examiner-Norman G. Torchin afl m. A m6 .m M.. PB .0 SN. r one as h m e m M .W D n 6 Assistant Examiner-J. P. Brammer [97], Pat.

ABSTRACT A cascading development method employs a devel- 6 5 M 3 O N operunit which is equipped with a developer pool at the upper portion of the development zone and a development electrode at the lower portion that provides high development efficiency, good solid area coverage and reduced background density.

[56] References Cited UNITED STATES PATENTS 2,927,554 3/l960 Oldenboom 118/637 5 Claims, 2 Drawing Figures I II liil I ll IIOIII II 1 ill PATENTED FEB 1 I975 SHEET 2 OF 2 XEROGRAPHIC DEVELOPMENT METHOD This is a division of application, Ser. No. 133,656, filed Apr. 13, 1971, now US. Pat. No. 3,741,156. This invention relates to imaging systems and in particular to novel methods for the development of electrostatic latent images in electrophotographic systems.

In Xerography, one form of an electrophotographic system, an electrostatic latent image is created on the surface of a photosensitive layer by uniformly depositing electrostatic charge on the surface of the photosensitive layer and by exposing the charged layer to a light image. The latent image can be developed by applying electroscopic particles to the latent image to form a visible image. The electroscopic particles adhere to the surface of the photosensitive layer in areas corresponding to the latent image. This visible or developed image comprised of electroscopic particles is thereafter transferred to a paper sheet or other transfer member and fixed to it by heat fusing or other technique to form a permanent copy.

The electroscopic particles can be applied to the latent image by passing a developer material comprising toner and carrier particles over the electrostatic latent image. The toner particles are the previously mentioned electroscopic particles and the carrier particles are much larger particles chosen to triboelectrically charge the toner particles so the latter are attracted toward or away from the charged photosensitive layer. The carrier particles can include core particles such as glass beads, sand and steel shot. These core materials can be coated with another material to have a triboelectric series ranking far removed from the ranking of the toner particles.

When the toner and carrier particles are mixed together the toner particles tend to adhere to the carrier particles because charge of opposite polarities are triboelectrically generated on the toner and carrier particles as a result of the mixing action. This mixture of toner and carrier is the developing material. When this developing material is passed over an electrostatic latent image the toner particles are attracted to the pho tosensitive layer because the electrical forces exerted by the charge associated with the lateral image is greater than the electrical and other forces holding the toner to the carrier.

Cascade development is an example of one way of passing the above developing material over the latent image. In cascade development, the mixture of toner and carrier is poured over the photosensitive layer, i.e., the moving force is that of gravity. Two mutually competitive mechanisms are involved with the cascade developing method. One mechanism is the depositing of toner onto the photosensitive layer in image areas and the other is the removal of toner from the image areas. The toner is deposited onto the photosensitive layer because of the electrical forces associated with the latent image. The toner is removed from the photosensitive layer because of the electrical forces between the carrier and toner. Generally, the forces acting on the toner that are associated with the latent image are greater than the forces associated with the carrier so that the net result is deposition of toner on the photosensitive layer in the vicinity of the latent image. The removal of toner by the forces associated with the carrier, both electrical and mechanical, is called the cleaning effeet. This cleaning effect is important because the toner deposited in background, i.e., non-latent image areas, can be removed. The toner is removed in both background and latent image areas but because the electric forces in image areas are strong and the electric forces in background areas are weak, the cleaning effect can be negligible in image areas and highly active in background areas.

Because of the existence of the foregoing competitive mechanisms, the mixing ratio of the toner and carrier particles should be carefully controlled. Excess toner concentration creates high deposition of the toner in background areas because of a low cleaning effect. On the other hand, scanty toner concentration gives rise to low image density because of excess cleaning effect. Therefore, it is possible to balance between image density and background density by adjusting the toner concentration in the developing material.

To overcome the disadvantages of cascade development, various methods have been proposed. An example of one method is where a developing electrode is spaced from the surface bearing the electrostatic latent image. In this case, a strong electric field is generated between the developing electrode and the latent image bearing surface. This strong field drives the toner toward the latent image and the carrier toward the developing electrode thereby minimizing the cleaning effect in image areas. In background areas, the fields are weak and the cleaning effect still is observed. The field between the photosensitive layer and developing electrode increases as the two are moved closer together. The practical limit on the spacing is determined by the size of the carrier. The carrier is quite large, as compared to the'toner, and can jam between the photosensitive layer and developing electrode. This jam, of course, can harm both the developed image and the photosensitive layer. Also, the developing material flow rate is too slow when the spacing is small which reduces the overall efficiency. Consequently, developing electrodes are normally spaced at a distance from the photosensitive layer that allows for reasonable flow rates without jams. The jam problem has been attacked by making the developing electrode porous or building it from wire mesh.

SUMMARY Accordingly, it is an object of this invention to overcome the above noted and other problems associated with cascade development.

It is another object of the present invention to enhance cascade development methods.

Another object of the instant invention is to devise a novel electrode configuration for cascade development.

These and other objects of the present invention are realized by providing a pool of developing material adjacent the latent image bearing surface and then cascading the developer over the latent image past a developing electrode. The developing material is continuously supplied to the pool and continuously flows from the pool into the space between the developing electrode and latent image bearing surface.

DESCRIPTION OF THE DRAWINGS These and other objects of the present invention will be apparent from a further reading and from the drawings which are:

FIG. 1 is a sectional view of a xerographic imaging machine having a photosensitive layer on a rotating drum and including cascade developing method and apparatus according to the present invention.

FIG. 2 is a sectional view of a xerographic imaging machine having a photosensitive layer on a continuous belt and including cascade developing method and apparatus according to the present invention.

DESCRIPTION OF THE EMBODIMENTS FIG. 1 shows a sectional view of a photosensitive drum 101 around which is positioned a charging station 102, an exposure station 103, a development station, transfer station 105, a cleaning station 106, and a uniform exposure station 107. Transfer sheets of paper 109 are held in a feed tray 108 positioned at the left hand side of the transfer station 105. The sheets are fed by feed rollers 111 and 112 to the transfer station 105 and by a transfer belt 1 13 through a fixing station 114.

The photosensitive drum 101 rotates around an axis in the direction indicated by the arrow. The drum comprises an insulating photoconductive layer, for example vitreous selenium layer 101b, formed on a cylindrical metal tube 101a.

The developer unit 1 comprises a casing 2, a developer pool 3 for the developer 9 defined by members including wall 4, a developing electrode plate 5, a bucket elevator 7 and a toner dispenser 8. The back wall 4 is positioned to face the surface of the drum 101, at a distance of (m) from the surface of the drum where (m) is a little shorter than the length (l) of the wall 4. The circular arc involving the angle 6 in the second quadrant of the drum 101, and the top end of the wall 4A bent to the left as shown in the drawing, facilitates catching developer poured onto bent wall 4A. Wall 4 is made of an electrically conductive material and the bottom end of the wall is connecting to the developing electrode plate through an electrically insulating spacer 10.

The developing electrode 5 is positioned close to and along the surface of the drum 101. The bottom end 5a of the wall 5 reaches a little below the horizontal line passing through the center 0. It is preferable to coat the surface, at least the areas with which the developer comes into contact, of wall 4 and electrode 5 with an electrically insulating resinous material. Any conductive material can be used for the developing electrode plate 5. The electrode plate 5 and the wall 4 of the pool 3 are electrically isolated from each other and voltages are applied to them independently.

At the center of the casing 2 is located the bucket elevator 7. The elevator comprises two driving rollers 7b and several buckets 7a that are joined to the belt 70. The buckets carry the developer 9 from the bottom 6 of the casing 2 up to the developer pool. The developer 9 comprises toner 9a and carrier 9b, the toner 9a being resinous fine particles of from about 0.1 to 30 mm in diameter and the carrier 9b being coarse particles of 30 to 800 mm in diameter. The toner dispenser 8 is positioned at the upper left corner of the casing 2 and drops the toner 9a to the bottom of the casing.

FIG. 2 shows a sectional view of a flexible photosensitive belt 201 supported by three rollers 200a, b and c. Positioned around the belt are charging station 202, exposure station 203, developing unit 21, transfer station 205, cleaning station 206 and uniform exposure station 207. The photosensitive belt 201 includes an insulating photoconductive layer 201b formed on a metal belt 201a. The developer unit 21 comprises a casing 22,

a developerpool 23 for the developer 9 defined by members including wall 24, a developing electrode plate 25, and a bucket elevator 27. The wall 24 of the pool 23 faces the surface of the photosensitive belt 201 at a distance (p) from the surface of the belt 201, where (p) is a little shorter than the length (n) of the wall 24. The top end 24a of the wall is bent to the left as shown in the drawing to facilitate catching developer poured from a bucket 27. The wall is made of an electrically conductive material the same as in embodiment of FIG. 1. The bottom end of the wall is connecting to the developing electrode plate 25 through an electrically insulating spacer 28.

The development electrode 25 is positioned close to and along the surface of the belt in a zone where the belt forms a circular arc. The bottom end 25a of the wall 25 extends to a point 30 below a horizontal line passing through the center 2000f the roller 200a. It is preferable to coat the surface, at least the area with which the developer comes to contact, of the wall 24 and the development electrode 25 with an electrically insulating resinous material. Any conductive material can be used for the development electrode plate 25. The electrode plate 25 and the wall 24 of the pool 23 are electrically isolated from each other and voltages are applied to them independently.

The bucket elevator 27 is positioned near the center of the casing and includes two main rollers 27a, two guide rollers 27b, and several buckets 27d that are joined to belt 27c. The buckets carry the developer 9 from the bottom 26 of the casing 22 'up to the developer pool 23. The toner 9a is supplied by a toner dispenser, not shown, that is similar to the dispenser used in the embodiment of FIG. 1.

In the embodiment of FIG. 1, a copy is made by steps including uniformly charging the rotating drum 101 at the charge station 102, exposing the charged surface to the image light at the exposing station 103, developing the latent image with the toner 9a in the developing unit 1, transferring the toner image from the surface of the photosensitive drum to a sheet of paper 109 fed out from the paper tray 108 at the transfer station 105 and fixing the toner image onto the sheet of paper at the fixing station 114. After passing the transfer station 105, the drum 1 01 is cleaned at the cleaning station 106 and then illuminated uniformly at the uniform exposure station 107 to be refreshed for the next use. I

The subject of this invention is the developing method and apparatus, so that the operation of development in accordance with this invention will be discussed hereinafter.

The fresh developer, which is supplied from the elevator bucket to the developer pool, accumulates in the developer pool against the drum surface bearing the electrostatic latent image. The motion of the developer in the pool is complicated. The developer in the pool flows downward under gravity and is further agitated and circulated by the motion of the drum. The flow speed of the developer is slower than that of the drum surface. The actual flow of the developer in the pool 3 is considered the combination of these two modes of flow, i.e., downwards-flow and rotating-flow. Further, the relative motion between carrier particles causes them to rotate and/or circulate. The above three motions increase the development efficiency because cascade development depends upon the number of or the frequency of collisions between carrier particles, which have toner particles on their surface, and the latent image bearing surface. A higher frequency of collisions yields greater efficiency with suitable development occurring in a shorter development time or over a smaller development zone or space. Also, coupling a voltage to wall 4 that supresses any powder clouds is helpful. In this case, toner never gets out the drum in the first instance.

In addition to the collision frequency, the development electrode effect can be obtained when the carrier particles comprise conductive cores. That is, the carrier itself when packed against the drum comprises a developing electrode. Obviously, the carrier particles are spaced closer than conventional developing electrodes, yet jamming will not occur.

Thus the developed image of the high quality can be obtained in a short development time or in a small development space. In the case of the high efficiency and high solid area coverage development, however, there is a disadvantage of deposition of the toner onto the background as described before. Namely, the developed surface which has the high density image and unfavorable background density of thetoner comes into the space under the development electrode 5. In the cascade development, in general, a powder cloud of the toner is generated by mutual collision of carriers and collision between carriers, and results in unfavorable deposition of the toner onto the background area. In the developer pool in this invention the generation of the powder cloud of the toner is relatively small. This unfavorable toner cloud may be eliminated by the development electrode 5 positioned below the pool 3. The cleaning effect is enhanced by applying a high voltage to the development electrode 5 to eliminate toner from being deposited onto the background area.

Though the development electrode is effective even in the third quadrant where the developer 9 flows apart from the latent image bearing surface by gravitational force, the more effective development may be achieved when the development electrode is in the second quadrant where the developer flows on the latent image bearing surface. The polarity of the applied voltage to the development electrode 5 is, of course, opposite to the polarity of the charge of the toner 9a.

In short, development of high efficiency is carried out in the developer pool and background toner is eliminated under the development electrode. The voltage of the latent image is applied to the wall 4 of the pool while the voltage of the background areas is applied to the developing electrode.

The operation and the function of the apparatus of FIG. 2 is substantially the same as that in FIG. 1. Consequently, the explanation of the machine in FIG. 2 is abridged.

To illustrate further the effectiveness of the development method in accordance with this invention the following examples are given.

EXAMPLE 1 An electrostatic latent image is formed on a photosensitive drum as shown in FIG. 1. The drum is prepared by vacuum coating about 50 mm vitreous selenium onto an aluminum cylinder. The latent image is formed by charging the drum to about 700 volts positive with a corona discharge and by exposing it to a light image.

The electrostatic latent image is developed in the de veloper pool 3 and background toner is removed by the developing electrode. The developer is comprised of one part by weight of toner particles made of black polystyrene copolymer of about 10 mm in diameter and parts by weight of coarse carrier particles of steel" shots having particle size of about 400 mm which were coated with a resinous material to provide toner particles negative triboelectric charges.

The developer fills more than 80 percent of the pool volume. The length of the circular arc of the developer pool wall 4 is about 50 mm, and the spacing (m) between the wall of the pool and the drum surface is about 30 mm. The wall is grounded as well as the drum.

As indicated in FIG. 1, the development electrode is positioned below the developer pool and is spaced from the drum surface by about 3 mm. The length of the developing electrode in the circular direction is about 35 mm. When development is carried out without the development electrode, the toner image has high image density and good solid black area coverage, but has high background density. When the developing electrode is in place but coupled to ground, no improvement in the background is seen. An image having high image density and reduced background is obtained when a positive 200 volts is applied to the developing electrode. When 400 volts is applied to the electrode nearly zero background density is obtained. When 800 volts is applied to the developing electrode, the image density is slightly reduced and the solid area coverage becomes poor.

The above experiment is done under a peripheral speed of about 15 cm/sec. for the drum.

When the drum is developed with a developing unit having no developer pool and no development electrode, the image is poor in solid area coverage and low in image density. In order to get the image density the same as in Example 1, the peripheral speed of the drum is reduced to 5 cm/sec. In this example, sufficient density is obtained even at a peripheral speed of 20 cm/sec.

EXAMPLE 2 An electrostatic latent image isformed on a photosensitive flexible belt, which is prepared by coating a vitreous selenium layer of about 40 um in thickness in vacuum on a flexible metal belt held with two rotating rolls having diameter of 200 mm and one roller of 50 mm diameter. The image is formed by charging the belt to about 700 volts positive with a corona discharger and exposing it to a light image. The belt on which the latent image is formed is subjected to the developer unit as indicated in FIG. 2. The developer used was same as in the example 1.

The length of the circular area of the developer pool in touch with the surface of the belt is about 55 mm, and the spacing (p) between the wall of the pool and the belt surface in FIG. 2 is about 30 mm. The wall and belt are grounded. As indicated in FIG. 2, the development electrode is positioned below the developer pool facing the belt surface at a spacing of 4 mm. The length of the electrode in the circular direction is about mm. In this case the bottom end of the electrode is positioned about 25 mm below the horizontal line passing through the center 200 of the roller. The image is excellent in image density and in solid area coverage with low background density.

EXAMPLE 3 The development electrode in example 2 is divided into two parts, the upper or the first electrode and the lower or the second electrode. The length of the first electrode in the circular direction is about 30 mm and that of the second is about 65 mm. Development is performed by applying a positive voltage of 200 volts to the first electrode and 600 volts to the second electrode under the same condition as in Example 2. The produced images are high in image density with uniform solid density and nearly zero background density.

EXAMPLE 4 A metal wire mesh is placed in the developer pool used in example 1 facing the photosensitive drum at a spacing of about 4 mm. The length of the wire mesh in the circular direction is about 30 mm. This is positioned in the upper part of the pool. The opening in the wire mesh are about 4 mm. The surface of the wire in the mesh is coated with an insulating resinous material. The mesh is isolated from the wall of the pool electrically and is coupled to an electric voltage. The developer used is a mixture of fine black particles of about l2 um in size of polystyrene copolymer and coarse sand particles of about 600 um in size which were coated with a resinous material to provide negative triboelectric charges to the fine particles. A developed image of high quality, i.e., of high image density, uniform solid area density and low background density, is obtained with this development unit when the positive voltage of 100 volts is applied to the metal wire mesh and 800 volts to the developer electrode. It is effective to vibrate the wire mesh during development.

As described before, the development electrode in this invention is not designed for enhancement of the development efficiency, but for cleaning or eliminating the unfavorable toner deposited on the background area. Therefore, it is not necessary to place the developing electrode especially close to the surface of the photosensitive layer to produce the electric field in high fidelity to the electrostatic latent image. A large electrode spacing can be used without any reduction in the cleaning effect in the development process. The large electrode spacing eliminates developer jam and ensures a sufficient developer flow rate. Thus with the method of this invention, high efficient development is obtained.

What is claimed is:

1. A method for developing a latent image on a movable photosensitive surface; said method comprising the steps of forming a developer pool substantially filling a first space between a conductive wall and said photosensitive surface, contacting the photosensitive surface with developer in said pool to develop said latent image, allowing excessive developer to flow from said first space into another narrower space between an electrode and said photosensitive surface, moving said photosensitive surface in the direction of developer flow at a rate which is faster than the flow of said devel oper through said first space, allowing said excessive developer to freely fall through said other space into a developer sump, and transporting developer from the sump to said first space.

2. The method of claim 1 wherein said conductive wall is adjacent to and electrically insulated from said electrode; and said conductive wall and said electrode are independently electrically biased to enhance image development and suppress background development, respectively.

3. A method for developing latent electrostatic images carried by a photosensitive surface while said surface is moving at a predetermined rate; said method comprising the steps of transporting developer from a sump to a space between a wall-like member and said photosensitive surface,

constricting a lower part of said space to cause developer to accumulate therein,

bringing developer in said space into contact with said photosensitive surface to develop any latent electrostatic images thereon, and

maintaining a relatively low flow rate for the developer in said space, whereby said photosensitive surface moves at a faster rate than said developer to thereby enhance the development of said images.

4. The method of claim 3 further including the step of discharging developer from said space to freely fall back to said sump along a path defined between an electrode and said photosensitive surface, said electrode being electrically biased to suppress background development.

5. The method of claim 4 wherein said wall-like member is an electrically conductive member and is electrically insulated from said electrode; and further including means for applying a predetermined potential to said conductive member. 

1. A METHOD FOR DEVELOPING A LATENT IMAGE ON A MOVABLE PHOTOSENSITIVE SURFACE, SAID METHOD COMPRISING THE STEPS OF FORMING A DEVELOPER POOL SUBSTANTIALLY FILLING A FIRST SPACE BETWEEN A CONDUCTIVE WALL AND SAID PHOTOSENSITIVE SURFACE, CONTACTING THE PHOTOSENSITIVE SURFACE WITH DEVELOPER IN SAID POOL TO DEVELOP SAID LATENT IMAGE, ALLOWING EXCESSIVE DEVELOPER TO FLOW FROM SAID FIRST SPACE INTO ANOTHER NARROWER SPACE BETWEEN AN ELECTRODE AND SAID PHOTOSENSITIVE SURFACE, MOVING SAID PHOTOSENSITIVE SURFACE IN THE DIRECTION OF DEVELOPER FLOW AT A RATE WHICH IS FASTER THAN THE FLOW OF SAID DEVELOPER THROUGH SAID FIRST SPACE, ALLOWING SAID EXCESSIVE DEVELOPER TO
 2. The method of claim 1 wherein said conductive wall is adjacent to and electrically insulated from said electrode; and said conductive wall and said electrode are independently electrically biased to enhance image development and suppress background development, respectively.
 3. A method for developing latent electrostatic images carried by a photosensitive surface while said surface is moving at a predetermined rate; said method comprising the steps of transporting developer from a sump to a space between a wall-like member and said photosensitive surface, constricting a lower part of said space to cause developer to accumulate therein, bringing developer in said space into contact with said photosensitive surface to develop any latent electrostatic images thereon, and maintaining a relatively low flow rate for the developer in said space, whereby said photosensitive surface moves at a faster rate than said developer to thereby enhance the development of said images.
 4. The method of claim 3 further including the step of discharging developer from said space to freely fall back to said sump along a path defined between an electrode and said photosensitive surface, said electrode being electrically biased to suppress background development.
 5. The method of claim 4 wherein said wall-like member is an electrically conductive member and is electrically insulated from said electrode; and further including means for applying a predetermined potential to said conductive member. 